Method of making high voltage metal oxide silicon field effect transistor

ABSTRACT

A manufacturing method of high voltage MOSFET includes a process forming the first and second conductive wells in a semiconductor substrate; process forming drift areas in the first and second conductive wells; process growing an isolation membrane on the substrate surface between the first and second conductive wells; process forming a gate insulation film; process forming a gate on the gate insulation film above the first and second conductive wells; process forming low concentration n-and p-type dopant areas in the drift areas of the parts adjacent to the gate; process forming buried diffusion areas in the first and second conductive wells; process forming source/drain having a body contact on a side on the buried diffusion areas in the first and second conductive wells; process forming an insulation film having a contact formed in such way that is exposed the surface of source/drain on the entire surface of the substrate including the gate and isolation membrane; process forming a metal film on the insulation film; and process forming source/drain electrodes and the metal field plates, by etching the metal film using a mask.

This application is a Divisional of application Ser. No. 08/756,433, nowU.S. Pat. No. 5,831,320, filed Nov. 26, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a structure and manufacturing method ofa power metal oxide silicon field effect transistor (hereinafterreferred to as "MOSFET"), and particularly to a structure andmanufacturing method of a high voltage MOSFET which obtains, a highbreakdown voltage with an area small enough to fit for the use of smartpower IC, and provided with a metal field plate to reduce anyspecific-on-resistance.

2. Description of the Prior Art

Since the power MOSFET has an excellent switching speed compared withother power elements, and a characteristic that it has low on-resistanceat any element of relative low resisting pressure less than 300(V), thehigh voltage lateral power MOSFET gets into the spotlight as powerelement for the very large-scale integrated circuit.

As power elements in general use, there are double-diffused MOSFET(DMOSFET), insulated gate bipolar transistor (IGBT), bipolar transistor,etc., but as complementary metal oxide semiconductor (CMOS) VLSI for lowvoltage, and elements for high voltage (10V to 500V), the lateraldouble-diffused MOSFET (hereinafter referred to as "LDMOSFET") isdeveloped as most effective element.

In FIG. 1(A) there is shown a sectional structure of a general LDMOSFETused widely for smart power IC. The manufacturing process of the saidLDMOSFET is described briefly as follows:

In the first place, a drift area (11) to prevent any high voltage ismade on a silicon substrate (13) by the epitaxial layer growth method orwell forming process using diffusion, and a field oxide (4) is formed onthe surface of substrate between the channel (8) and the drain (10)using the process of the local oxidation of silicon.

In the next step, an oxide film is grown as gate-insulating film on theentire surface of the substrate including the said field oxide(4), andthen n+ polycrystalline silicon film is formed by implementing POCl3diffusion after depositing the polycrystalline silicon film on the saidoxide film.

In the next step, a polycrystalline silicon gate (1) is formed byselecting and etching the said N+ polycrystalline silicon film using aphotosensitive mask, and after self-align to the polycrystalline silicongate on the side of source, and ion-injection of p-dopants, the drive-inis performed. As a result, a double-diffused well (or p-well) is formed.

In the continuous step, a source(6) and a drain(10) are formed by anion-injection of n-dopants in the specified parts of the right and leftsides of the said polycrystalline silicon gate(1), and p+ area, a bodycontact (5) is formed at a location adjacent to the said source(6), byimplementing the ion-injection of p-dopants after forming aphotosensitive mask on the said source(6).

Subsequently, an oxide film and an insulation film are deposited to thesubstrate including the said polycrystalline silicon gate(1), and thesurface of such oxide film is evened by reflowing it at the temperatureof 900 C. to 1000 C., and the ion-injected dopants are activatedsimultaneously.

Then, a contact hole is formed by etching the oxide film with a mask forforming the contact, to the extent that the specified part of the saidsource(6) and the specified parts of the body contact(5) and drain areexposed, and after depositing a metal film over the entire surface ofoxide film including the said contact hole, a metal lead, for example, asource electrode(2) and a drain electrode(3) are formed by selecting andetching it, respectively.

Finally, the manufacture of elements are completed by depositing aninsulation film, for example an oxide film as a passivation layer toprotect the element, over the entire surface of the oxide film includingthe said source and drain electrodes 2, 3), and opening then the pad.

Accordingly, in case of n-channel high voltage LDMOSFET, an inversionlayer is formed in the channel area(8) when a voltage higher than thethreshold voltage is applied, and if a voltage higher than that of thesource electrode(2) terminal is applied at that time to the drainelectrode(3) terminal, the electrons are supplied from source(6) tochannel(8), through the surface drift area(9) in the lower part of thefield oxide(4), to the drain(10), and which makes the current flow.

Although the said elements may be employed in diversified ways in highside driver (HSD), low side driver (LSD) and H-bridge circuits withinchips, and it is easily manufactured, there are disadvantages that thesubthreshold slope is so large due to ununiformity of the dopingconcentration in the channel area which is the structure of LDMOSFETitself, that the threshold voltage is risen, and that a breakdownphenomenon occurs on the silicon substrate surface in drift area nearthe channel.

In FIG. 1(B) is shown a structure of a reduced surface field (RESURF)LDMOSFET in which the performance of LDMOSFET as described in FIG. 1(A)is improved.

In comparison with drift area made by the LDMOSFET as described in FIG.1(A) on substrate using the epitaxial growth method or well formationprocess, so as to include all the parts in which source, drain and fieldoxide(4) are formed, in the said element, the drift area is formed so asto have the minimum area, by implementing ion-injection and drive-in orepitaxial layer formation in parts in which field oxide(4) and drain(10)are formed, in a manner that such parts are adjacent to the saidD-well(7), as shown in FIG. 1(B) to use the RESURF principles.

To form the elements by the said way is to improve the breakdownphenomenon and conduction resistance on the silicon substrate surfacewhich are disadvantages of the LDMOSFET as shown in FIG. 1(A), and hasadvantages that it may obtain high breakdown voltage and low conductionresistance with the minimum area, by forming the drift layer aswell-type or epitaxial layer.

However, such an element has disadvantages in that it may be used onlyas LSD, for p-type substrate(12) is connected to the source and the bodycontact(5), and it being of the DMOS structure, it is inevitable to havea high threshold voltage due to ununiformal doping concentration of thechannel.

In FIG. 1(C) is shown the RESURF extended drain MOSFET (EDMOSFET) forimproving the threshold voltage characteristics of such LDMOSFET andeliminating the restriction on the scope of application which is ashortcoming of the RESURF LDMOSFET.

The said element has no D-well (or p-well) as shown in FIGS. 1(A) and1(B).

Since the doping concentration in the channel area is uniform, thethreshold voltage may be r educed, and for the purpose of obtaining anydesires threshold voltage, it is possible to adjust the thresholdvoltage to a desired one by carrying out a threshold voltage adjustmention-injection into the channel area(8), as in the low voltage MOSFET.

In case of n-channel high voltage RESURF EDMOSFET, therefore, the mosthigh voltage applied to the drain electrode(3) terminal is applied tothe drift area(11), and some voltage is applied to the channel area(8).In this case, when a voltage higher than the threshold voltage isapplied to a polycristalline silicon gate(1), an inversion layer isformed in the channel area(8), and when a voltage lower than that of thedrain electrode(3) terminal is applied to the source electrode(2)terminal, electrons flow from the source(6) through the channel area(8)and drift area(11) to the drain(10).

As reported by O. K. Kwon et al., "Optimized 60 V Lateral DMOS Devicefor VLSI Power Applications," 1991 Symposium on VLSI Technology, Oiso,Japan, pp.115-116, however, the above-mentioned element has alsodisadvantages in that since the electrons having passed through thechannel area(8) flow below the field oxide transiting the surface driftarea(9), and the current conduction path is distorted, a high conductionresistance is inevitable.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a structureand manufacturing method of a high voltage MOSFET designed to embody ahigh breakdown voltage and low conduction resistance in the minimumarea, by forming a metal field plate in the course of a metal electrodeformation process.

The first structure of high voltage MOSFET according to the presentinvention to achieve the above-mentioned object includes a semiconductorsubstrate; first and second conductive wells formed in the saidsubstrate; isolation membrane formed on substrate between the said firstand second conductive wells; gates formed on surfaces of the substrateson which the said first and second conductive wells are formed,respectively; drift areas formed within the said first and secondconductive wells, respectively, so as to be connected to lower part ofone edge of the said gate; drains formed in the said drift areas,respectively; source having a depressed diffusion area formed in thesaid first and second conductive wells, respectively, so as to beconnected to the lower part of the other edge of the said gate; bodycontacts formed in the said first and second conductive wells,respectively, and formed on a side of the said source having a depresseddiffusion area; first insulation film formed on substrate including thesaid gate and isolation membrane, and having a contact hole formed sothat surfaces of the source and drain are exposed; source/drainelectrodes formed in specified parts of the first insulation filmincluding the said contact hole, respectively; field plates located inthe said drift area and on the first insulation film on the gate,respectively, and formed between the said source/drain electrodes so asto be separated from them; and second insulation film formed on thefirst insulation film including the said source/drain electrodes and thefield plates.

The second structure of the high voltage MOSFET according to the presentinvention to achieve the said object includes semiconductor substrate;first and second conductive wells formed in the said substrate;isolation membrane formed on the substrate between the said first andsecond conductive wells; gates formed on surfaces of substrates on whichthe said first and second conductive wells are formed, respectively;first and second drift areas formed in the said first conductive well soas to be connected to lower parts of both edges of the said gates,respectively, in a form separated from each other at a specifieddistance; first and second drift areas formed in the said secondconductive well so as to be connected to lower parts of both edges ofthe said gates, respectively, in a form separated from each other at aspecified distance; source/drain formed in the first and second driftareas formed in the first and second conductive wells, respectively;body contacts formed in the said first and second conductive wells, andformed on each side with am isolation membrane between the first andsecond drift areas, respectively; first insulation film formed onsubstrate including the said gates and insulation film, and having acontact hole formed so that surfaces of source/drain and body contactare exposed; source/drain electrodes formed in specified parts of thefirst insulation film including the said contact hole; first and secondfield plates located on the first insulation films in and on the saiddrift areas and gates, respectively, and formed between the saidsource/drain electrodes so as to be separated from them; and secondinsulation film formed on the first insulation film including the saidsource/drain electrodes and the first and second field plates.

The third structure of the high voltage MOSFET according to the presentinvention to achieve the said object includes semiconductor substrate;first conductive well formed in the said substrate; gate formed onsubstrate surface on which the said first conductive well is formed;drift area formed in the said first conductive well so as to beconnected to lower part of one edge of the said gate; drain formed inthe said drift area; source having a depressed diffusion area formed inthe said conductive well so as to be connected to lower part of theother edge of the said gate; body contact formed in the said firstconductive well, and formed on one side of the said source having adepressed diffusion area; first insulation film formed on substrateincluding the said gate, and having contact hole formed so that surfacesof the source and drain are exposed; source/drain electrodes formed inspecified parts of the first insulation film including the said contacthole; field plates located in and on the said drift area and the firstinsulation film on the said gate, and formed between them so as to beseparated from them; and second insulation film formed on the firstinsulation film including the said source/drain electrodes and gate.

The fourth structure of the high voltage MOSFET according to the presentinvention to achieve the said object includes semiconductor substrate;first conductive well formed in the said substrate; gate formed onsubstrate surface on which the said first conductive well is formed;first and second drift areas formed in the said first conductive well soas to be connected to lower parts of both edges of the said gate, in aform separated from each other at a specified distance; source/drainformed in the first and second drift areas within the said conductivewell; body contacts formed in the said first conductive well, and formedon each side with an isolation membrane between the first and seconddrift areas, respectively; first insulation film formed on substrateincluding the said gate and isolation membrane, and having a contacthole formed so that surfaces of the source/drain and body contactsurfaces are exposed; source/drain electrodes formed in specified partsof the first insulation film including the said contact hole; first andsecond field plates located in and on the said drift areas and on thefirst insulation film on gate, and formed between the said source/drainelectrodes so as to be separated from them; and second insulation filmformed on the first insulation film including the said source/drainelectrodes and the said first and second field plates.

Meanwhile, the first manufacturing method of the high voltage MOSFETaccording to the present invention to achieve the said object, ischaracterized in that it comprises process forming the first and secondconductive wells in a semiconductor substrate; process forming driftareas in the said conductive wells, respectively; process growing anisolation membrane on substrate surface between the said first andsecond conductive wells; process forming gate insulation film; processforming gates on the gate insulation films on the said first and secondconductive wells; process forming low concentration n-and p-type dopantareas in drift areas in parts adjacent to the said gates; processforming depressed diffusion areas in the said first and secondconductive wells; process forming source/drain having body contactformed on a side in the depressed diffusion area within the said firstand second conductive wells; process forming insulation film havingcontact formed so that surface of source/drain is exposed on the entiresurface of substrate including the said gate and isolation membrane;process forming a metal film on the said insulation film; and processforming source/drain electrodes and metal field plates by etching thesaid metal film using mask.

The second manufacturing method of the high voltage MOSFET according tothe present invention to achieve the said object, is characterized inthat it comprises process forming the first and second conductive wellsin semiconductor substrate; process forming the first and second p-typedrift areas within the said first conductive well; process forming thefirst and second n-type drift areas within the said second conductivewell; process growing an isolation membrane on substrate surface betweenthe said first and second conductive wells; process growing oxide filmson both edges of the said first and second p-type drift areas and thesaid first and second n-type drift areas; process forming a gateinsulation film; process forming gates on the gate insulation filmbetween the said first and second n-type drift areas and on the gateinsulation film between the said first and second p-type drift areas;process forming the n-and p-type low concentration dopant areas in thefirst and second p-type drift areas in parts adjacent to the said gatesand in the first and second n-type drift areas; process formingsource/drain areas in the said first and second p-type drift areas andthe said first and second n-type drift areas, and forming simultaneouslybody contacts on both sides of oxide film; process forming insulationfilm having a contact formed so that the source/drain areas and thesurface of body contact are exposed over the entire surface of substrateincluding the said gates and isolation membrane; process forming metalfilm on the said insulation film; and process forming source/drainelectrodes and the first and second field plates by etching the saidmetal film using mask.

The third manufacturing method of the high voltage MOSFET according tothe present invention to achieve the said object, is characterized inthat it comprises process forming the first conductive well insemiconductor substrate; process forming a drift area in the said firstconductive well; process forming a gate insulation film on substrate onwhich the said drift area is formed, and then forming a gate on it;process forming low concentration n-or p-type dopant area in the driftarea in part adjacent to the said gate; process forming a depresseddiffusion area in the said first conductive well; process formingsource/drain having body contact on a side in the depressed diffusionarea within the said first conductive well; process forming insulationfilm having a contact formed so that source/drain surface is exposed onthe entire surface of substrate including the said gate; process forminga metal film on the said insulation film; and process formingsource/drain electrodes and metal field plate by etching the said metalfilm using mask.

The fourth manufacturing method of the high voltage MOSFET according tothe present invention to achieve the said object, is characterized inthat it comprises process forming the first conductive well insemiconductor substrate; process forming the first and second driftareas in the said first conductive well; process growing oxide films onboth edges of the said first and second drift areas; process forming agate insulation film; process forming a gate on the gate insulation filmbetween the said first and second drift areas; process forming lowconcentration n-or p-type dopant areas in the first and second driftareas in parts adjacent to the said gate; process forming source/drainareas in the said first and second drift areas, and formingsimultaneously body contacts on both sides of the oxide film; processforming insulation film having a contact formed so that source/drainareas and body contact surface are exposed on the entire surface ofsubstrate including the said gate; process forming metal film on thesaid insulation film; and process forming source/drain electrodes andfirst and second metal field plates by etching the said metal film usingmask.

It is possible as a result of manufacture of elements in the saidstructure, to maintain the advantages of the existing RESURF EDMOSFET,and to prevent the current conduction path from being distorted by fieldoxide.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, the preferred embodiments ofthe present invention will hereinafter be described in details.

FIGS. 1(A) to 1(C) show sectional structures of the MOSFET for the smartpower IC according to the conventional art, in which FIG. 1(A) is across-sectional view showing a structure of the LDMOSFET;

FIG. 1(B) is a cross-sectional view showing a structure of the RESURFLDMOSFET; and

FIG. 1(C) is a cross-sectional view showing a structure of the RESURFEDMOSFET;

FIGS. 2(A) and 2(B) show a structure of the RESURF EDMOSFET according tothe present invention, in which FIG. 2(A) is a cross-sectional viewshowing a structure of N-channel RESURF EDMOSFET; and

FIG. 2(B) is a cross-sectional view showing a structure of P-channelRESURF EDMOSFET;

FIGS. 3(A) to 3(N) are process flow charts showing manufacturing methodof the RESURF EDMOSFET described in FIG. 2;

FIGS. 4(A) and 4(B) are comparative illustrations on distribution ofvoltage up to the maximum voltage applied between drain and source, incase where a metal field plate is or is not formed on the RESURFEDMOSFET as described in FIG. 2, in which

FIG. 4(A) is a view showing an equivalent potential line of voltage upto the maximum yield voltage, in case where a metal field plate isformed (in case where the yield voltage is 101.5(V), and the voltageapplied to the drain is 100(V)); and

FIG. 4(B) is a view showing an equivalent potential line of voltage upto the maximum yield voltage, in case where a metal field plate is notformed (in case where the yield voltage is 68(V), and the voltageapplied to the drain is 70(V));

FIGS. 5(A) and 5(B) are comparative illustrations on a currentconduction path, in case where a metal field plate is or is not formedon the RESURF EDMOSFET as described in FIG. 2, in which

FIG. 5(A) is a view showing the current conduction path, when the samevoltage as gate is applied to the metal field plate, in case where themetal field plate is formed; and

FIG. 5(B) is a view showing the current conduction path, in case wherethe metal field plate is not formed;

FIG. 6 is a cross-sectional view showing a structure of n-channelbi-directional ESURF EDMOSFET according to the present invention; and

FIGS. 7(A) to 7(E) are process flow charts showing the manufacturingprocess of n-channel bi-directional RESURF EDMOSFET as described in FIG.6.

DETAILED DESCRIPTION OF THE INVENTION

In case of an ideal power MOSFET, a perfect RESURF effect (phenomenonthat substrate surface field is reduced) should be created to realizethe maximum yield voltage in the minimum area, and in case of theconduction resistance, it would draw close to the ideal value, only whenno distortion exists in a current conduction path at the time theperfect RESURF effect is created.

However, since the yield voltage and the conduction resistance are tocompromised each other upon manufacturing actually the elements, it isvery difficult to design them in such manner that the yield voltagebecomes maximum, and the low conduction resistance is realizedsimultaneously.

The present invention proposes in consideration of such matters, a newstructure of RESURF EDMOSFET having a high yield voltage in the minimumarea, and capable of realizing a low conduction resistance by excludingthe current distortion path, with collecting advantages of the existingLDMOSFET, RESURF LDMOSFET, and RESURF EDMOSFET.

This new structure of RESURF EDMOSFET is characterized in that the fieldplate being formed with a metal, no additional manufacturing process isrequired, it is adjustable so as to hold a low threshold voltage, and itmay be used in diverse application circuits.

In FIGS. 2(A) and 2(B) are shown n-and p-channel structures of theRESURF EDMOSFET having the said characteristics, respectively.

The method for embodying actually n-and p-channels of the said structureon a semiconductor substrate, is described concretely as follows, byreference to the process flow charts shown in FIGS. 3(A) to 3(H).

As shown in FIG. 3(A), a nitride hard mask is first formed in the mannerthat the substrate surface of the part on which n-well(21) is to beformed, is exposed on n-or p-type silicon substrate which is asemiconductor substrate(13), and n-type dopants are ion-injected usingthe said mask. After growing a field oxide on the substrate surface ofthe part on which n-well is to be formed, the substrate surface of thepart on which p-well(12) is to be formed, is exposed by removing thesaid nitride hard mask, and after ion-injection of p-type dopants in theexposed part of substrate using the said field oxide as mask, n-well(21)and p-well(12) which are active wells (12, 21), by implementing adrive-in in such state, and then the said field oxide is removed.

Since the field oxide is grown in a process forming n-well(21), thesurface of silicon substrate is not uniform as seen in the said figure,and a part of the silicon substrate in the area where the field oxide isgrown, is etched, and a step is made thereby.

Then, as shown in FIG. 3(B), after a mask is formed in a manner that aspecified part of the said p-well(12) surface is exposed, n-type dopantsare ion-injected into such exposed part, and then the said mask isremoved.

Continuously, after a mask is formed in a manner that a specified partof the said n-well(21) surface is exposed, p-type dopants areion-injected into such exposed part, and then the said mask is removed.In this state, the drift areas (11, 20) to which most high voltage isapplied, are formed by implementing the drive-in at a proper temperatureand time. Accordingly, n-type drift area(11) is formed at the saidp-well(12), and p-type drift area(20), at the said n-well(21),respectively.

Subsequently, a thin thermal field oxide(c) is formed in thickness ofabout 400 Z, to isolate elements, as shown in FIG. 3(C), after a nitridefilm is deposited on it, the nitride film of the part adjacent to thesaid p-and n-wells(12, 21) is etched using the mask, and then n-channelfield stop ion-injection is carried out using it as mask.

Then, a field oxide (a) is grown by the LOCOS method, and the saidnitride film is removed. In such process, n-channel field stopion-injection area is activated concurrently to form p+area in lowerpart of the said field oxide (a), and it is thereby possible to isolateelectrically n-and p-channel elements.

As shown in FIG. 3(D) and 3(E), the ion-injection for adjusting thethreshold voltage(Vt) is implemented using photosensitive film (b) asmask, and then the photosensitive film (b) at the left and right of thefield oxide for isolation of elements, formed in the said p+area, andthe thin field oxide (c) are removed. In FIG. 3(D) is shown a processcarrying out ion-injection for adjusting the threshold voltage ofn-channel elements, and in FIG. 3(E) is shown a process carrying oution-injection for adjusting the threshold voltage of p-channel elements.

Continuously, gate field oxides(22) are grown by a thermal oxidationprocess on substrate surface at the left and right of the field oxidefor isolation of elements.

As the gate field oxide (22) is grown, the boron forming p-type drift(20) area is segregated into the said field oxide (22), which results ina drop of doping concentration on surface. In this case, the phosphorusforming n-well (21) is diffused over area in which such dopingconcentration is low, so that p-type drift area (20) is distorted in itsform. For the similar reason, in n-type drift area (11), the boronforming p-well (12) is segregated into the said field oxide (22), sothat the doping concentration of p-well (12) on surface becomes low.Therefore, the phosphorus in n-type drift area (11) is diffused overthis area, and there happens a pile-up phenomenon.

In such state, N+polycristalline silicon is formed by implementing POCl3doping, after depositing the polycristalline silicon on the said gatefield oxide (22), and then N+polycristalline silicon gate (1) is formedby etching it using a mask.

As shown in FIG. 3(F), a mask layer (in the figure, the part indicatedby slant lines) is formed on the field oxide (22) including the saidN+polycristalline silicon gate (1), in such manner that a specified partof the field oxide (22) formed on n-type drift area (11) on the sideadjacent to the gate(1) is exposed, and after n-type lightly doped driftarea (23) is formed by self-align at the edge of the gate, withion-injection of low concentration n-type dopants into such exposedpart, the said mask layer is removed.

Then, as shown in FIG. 3(G), a mask layer (in the figure, part indicatedby slant lines) is formed on the field oxide (22) including the said N+polycristalline silicon gate (1), in such manner that a specified partof the field oxide (22) formed on p-type drift area (21) on the sideadjacent to the gate (1) is exposed, and after forming by self-alignp-type LD drift area (24) by ion-injection of low concentration p-typedopants into such exposed part, the said mask layer is removed.

As shown in FIG. 3(H), n- and p-type drift areas (11, 20) are formedadditionally on the side adjacent to the gate (1) by implementing adiffusion process.

To form double the drift areas in this way is to prevent any causablefactors of defective elements due to misalign in the course of process,for example, a phenomenon, such as short between the said gate (1) andeach drift area (11, 20), and the like. Upon advancing the process asabove-described, the length of the gate (1) may be reduced considerablycompared with any existing elements.

It is the reason that the length of existing elements is formed takinginto consideration even defective element factors due to error inprocess, while the drift area is formed according to the presentinvention in a form of more or less extended length in horizontaldirection through a double diffusion process, and it is possible as aresult to reduce the gate length by the increment caused by a processerror.

As an example, in case of the RESURF EDMOSFET proposed by the presentinvention, it is possible to form the length of gate in the limit of 1.2μm to 1.5 μm, on the basis of 100(V) elements, and a reduction in sizeof elements enables the area occupied by such elements in a system to bereduced so much.

In order to restrain a latch-up phenomenon caused by parasitic bipolartransistors, high concentration n-type dopants is first ion-injected byhigh energy using a mask, after self-aligning to N+ polycristallinesilicon gate (1) of p-channel element, and then p-type dopants in highconcentration is ion-injected by high energy using the mask afterself-aligning to N+ polycristalline silicon gate (1) of n-channelelement, and finally after activating them, a buried N+ area (16) isformed in n-well (21) and a buried p+ area (7), in p-well (12),respectively, as shown in FIG. 3(I).

Subsequently, as shown in FIG. 3(J), a mask layer (in the figure, partindicated by slant lines) is formed over the entire surface of thesubstrate (13) including n+ polycristalline silicon gate (1), and afteretching it in such way that a part of the field oxide surfaces on theburied n+ area (16), p+ area (7) and the surface of n-type drift area(11) is exposed, as shown in the said figure, N+ source/drain ision-injected into such exposed part, and then the said mask layer isremoved.

Then, as shown in FIG. 3(K), a new mask layer (in the figure, partindicated by slant lines) is formed over the entire surface of thesubstrate including N+ polycristalline silicon gate (1), and afteretching it in such way that a part of the field oxide surfaces on thesaid buried N+ area (16) and p+ area (7) and the surface of p-type driftarea (20), is exposed as shown in the said figure, p+ source/drain ision-injected into such exposed part, and then the said mask layer isremoved.

As shown in FIG. 3(L), the said ion-injected area is activated. As aresult, N+ SOS (6) having p+ body contact (5) formed so as to beadjacent to one side, is formed in the buried p+ area (7), and N+ typedrain (10) is formed in n-type drift area (11), while p+ source havingN+ body contact (14) formed so as to be adjacent to one side, is formedin the buried N+ area (16), and p+ drain (19) is formed in p type driftarea (21). That is, n-channel is formed in the said p-well (12), andp-channel is formed in the said n-well (20).

And then, as shown in FIG. 3(M), after depositing a silicon field oxide(25) as insulation film over the entire surface of substrate includingthe said gate, and etching it in such way that a specified part of thesource and drain using a mask, the contact is formed. Subsequently, ametal film for forming electrodes is deposited over the entire surfaceof the silicon field oxide (25) including the said contact, and afterselecting and etching it, the source/drain electrodes (2, 3) and themetal field plate (4a) are formed.

Finally, as shown in FIG. 3(N), this process is completed by depositingthe field oxide (26) which is a passivation film, over the entiresurface of the silicon field oxide (25) including the said source anddrain electrodes (2, 3) and metal field plate (4a), and etching thefield oxide on the pad.

In this case, the limit of the application yield voltage of the saidhigh voltage EDMOSFET is 20V to 600V, which is obtainable by changingthe well doping profile, drift doping profile, lengths of drift area andgate, under the optimum condition.

On the other hand, the said manufacturing process may be implementedeven by a single well mode, not by the twin well mode asabove-described, but it proceeds in the same way as in FIG. 3, withexception that only one well is formed selectively, and the detaileddescription thereof is omitted.

The operation of the RESURF EDMOSFET manufactured through such a seriesof process is described as follows, but since n-and p-channel RESURFEDMOSFET is operated by the same principle, only the operation ofn-channel RESURF EDMOSFET is described here.

The operation of n-channel RESURF EDMOSFET is described as follows: whena voltage higher than the threshold voltage is applied to N+polycristalline silicon gate (1), and a high voltage compared with thevoltage of source electrode (2) terminal is applied to a terminal ofdrain electrode (3), electrons flow in the drain (10) from the source(6) through the channel area (8) and part (9a) of the drift area,without distorting the current conduction path. In such process, themetal field plate (4a) prevents any breakdown phenomenon from occurringat the end of gate on a side near the drain (10), the said elementenables the breakdown voltage to be increased, and when any adequatevoltage is applied to the metal field plate (4a), the current conductionpath in the drift area can be improved, and which results in improvementof the conduction resistance.

For instance, in case of an optimal design of 100(V) n-channel EDMOSFETusing the above-described structure, the breakdown voltage is 101.5 (V),and the conduction resistance is 1.14 m cm2, which, proved as a resultof experiments, represent the most excellent characteristics ashorizontal power elements reported until now.

It is possible that the said RESURF EDMOSFET is driven by the followingtwo ways depending on the characteristics demanded by users. The one isto increase the breakdown voltage and to improve at the same time thecharacteristic of the conduction resistance, by applying the voltage ofgate (1) to the metal field plate (4a), and the other, to reduce theconduction resistance by applying any specified voltage different fromthat of gate(1) to the metal field plate (4a).

In FIG. 4(A) and 4(B), the distribution of voltage is shown incomparison with each other, when the maximum breakdown voltage isapplied between the source and drain of the element, in cases where themetal field plate is and is not formed at the RESURF EDMOSFET asdescribed in FIG. 2.

FIG. 4(A) shows a distribution of the voltage equipotential line up tothe maximum breakdown voltage, when the breakdown voltage is 101.5(V),and the voltage applied to the drain is 100(V), in case where the metalfield plate is formed.

In this case, as the maximum field vector is leaded towards the fieldoxide of gate at the end of the polycristalline silicon gate (1) on thedrain side, as shown in the said figure, the equipotential line ofvoltage continues to expand towards the drain area (10).

On the other hand, FIG. 4(B) shows a distribution of voltageequipotential line up to the maximum breakdown voltage, when thebreakdown voltage is 68(V), and the voltage applied to the drain is70(V), in case where the metal field plate is not formed.

In this case, since there is no metal field plate as shown in the saidfigure, the equipotential lines are collected at the end ofpolycristalline silicon gate (1) on the drain side, and allequipotential lines are thereby expanded towards the drain area (10), sothat a breakdown phenomenon occurs at the end of the polycristallinesilicon gate (1) on the drain side.

That is to say, it becomes apparent from the said result, that thebreakdown voltage may be increased by using a metal field plate.

In FIG. 5(A) and 5(B), the current conduction paths are shown incomparison with each other in cases where a metal field plate is and isnot formed.

FIG. 5(A) shows a case where a metal field plate is formed, in which itis shown detailed how the current conduction path is changed as a propervoltage is applied to the metal field plate. In this case, it shows theshape of the current conduction path made in drift area by applying anequal voltage to the metal field plate together with the gate (1).

On the other hand, FIG. 5(B) shows as a linear area the currentconduction path, in case where a metal field plate.

In FIG. 5(A), when a proper voltage is applied to the metal field plate,the current conduction path becomes narrower in the position (9a) inFIG. 2, compared with the case where the metal field plate is notformed. However, a carrier constituting the current flowing in the driftarea below such metal field plate, concentrates and flows on the surfaceof drift area, so that the current conduction path gets short, and theconduction resistance may thereby be reduced. In case of n-channelelement, as a high positive voltage is applied to a source on the metalfield plate, and in case of p-channel element, as a high negativevoltage is applied to a source on the metal field plate, electrons areaccumulated so much in the position (9a) of FIG. 2 in case of n-channelelement, and holes, in case of p-channel element, respectively, that thecurrent in the linear area, by which the conduction resistance isdetermined, is increased still more.

The said RESURF EDMOSFET is not only driven in a uni-direction, but inbi-direction. If it is desired to make a bi-directional drive, it issufficient to form a drift area even at the end of source in thesubstrate. FIG. 6 shows in sectional view a structure of such n-channelbi-directional RESURF EDMOSFET. As above-mentioned, the structure ofp-channel bi-directional RESURF EDMOSFET is substantially identical tothe basic structure of n-channel element, except only dopants dopingwith n-channel elements, and the sectional view thereof is here omitted.

As it is obvious in the said figure, the bi-directional RESURF EDMOSFETis principally identical in structure to the uni-directional RESURFEDMOSFET, with an exception that the drift area (11) existing only inthe drain is made even at the end of source, so that if a high voltageis applied to the drain or source without distinction, the one ofterminals thereof may play a role of the drain and the other, a role ofthe source, respectively.

In this case, since the source is not fixed to the terminal of bodycontact (5), it is formed separately at both ends of the drift area (11)as shown in the said figure, by means of ion-injection of any substancesuch as the said well (12). p+ area in the said figure corresponds toit.

The manufacturing method of n-channel RESURF EDMOSFET is describedbriefly by reference to a process flow chart as shown in FIG. 7(A) to7(E). The manufacturing method of p-channel RESURF EDMOSFET takes aprocess similar to the manufacturing process of n-channel RESURFEDMOSFET, and the description on the manufacturing method of the formeris here omitted.

In order to form an active area for inserting n-channel element of highvoltage in p-type substrate (13), as shown in FIG. 7(A), p-well (12) isformed through p-type dopants ion-injection and diffusion process, andthen the first and second n-type drift areas (11a, 11b) are formed inthe said p-well (12) using a mask, by implementing again n-type dopantsion-injection and diffusion process.

Then, as shown in FIG. 7(B), a thick field oxide is grown at both edgesof the first and second drift areas (11a, 11b), by applying the LOCOSprocess.

Subsequently, as shown in FIG. 7(C), a gate field oxide (22) is grown byapplying thermal oxidation process, on the substrate between the saidfield oxides, polycrystalline silicon is deposited on it, N+polycrystalline silicon layer is formed by POCl3 doping, and etchedselectively using a mask to form the gate (1) on the substrate betweenthe first and second drift areas (11a, 11b).

In this case, as a gate field oxide (22) is grown, the boronconstituting p-well penetrates into the said field oxide, and the dopingconcentration of p-well becomes low on the surface, and phosphorus inn-type drift area (11) diffuses over this area, and a pile-up phenomenonoccurs, as described for the foregoing process, so that the drift areahas such shape as shown in the figure.

In case of p-channel element, since there occurs a phenomenon accordingto the same principle, that the boron constituting p-type drift area(20) penetrates into the field oxide (22), and the doping concentrationbecomes low on the surface, and thereby the phosphorus forming n-welldiffuses over lower doping concentration area, so that any distortion ofa form as shown in FIG. 3(F) takes place in p-type drift area.

As shown in FIG. 7(D), a mask layer is formed on the field oxide (22)including the said n+ polycrystalline silicon gate (1), in such way thata specified part of the field oxide (22) formed on n-type drift area(11) on the side adjacent to the gate (1), is exposed, and after formingn-type lightly doped (LD) ion-injection area (23) by ion-injection oflow concentration n-type dopants in such exposed part, the said masklayer is removed, and a n-type drift area is formed additionally on theside adjacent to the gate (1), by implementing the diffusion process.

As shown in FIG. 7(E), the source/drain (6, 10) is formed byion-injection and diffusion of high concentration n-type dopants in thesaid first and second n-type drift areas, p+ body contact (5) is formedby ion-injection and diffusion of high concentration p-type dopants onboth sides of the field oxide formed on both edges of the said first andsecond drift areas (11a, 11b), and then the gate field oxide (22) on thepart excluding lower part of the said N+ polycrystalline silicon gate(1) is removed.

Continuously, a silicon field oxide (25) is deposited as insulation filmon the entire surface of the substrate including the said gate (1), andthen a contact is formed by etching it using a mask, in such way that aspecified part of the source/drain (6, 10) and p+ body contact (5) isexposed.

Subsequently, this process is completed by depositing metal film forforming electrodes over the silicon field oxide (25) including the saidcontact, forming by selecting and etching it, source and drainelectrodes (2, 3) and the first and second metal field plates (4a), andthen depositing the field oxide (26) which is a passivation film, overthe entire surface of the silicon field oxide (25) including the saidsource and drain electrodes (2, 3) and the first and second metal fieldplates (4a).

It is possible to carry out the said manufacturing process by the singlewell mode different from the twin well mode as above-described, and itis identical to the process as described in FIG. 7, except in that onlya single well is formed by the process as described in FIG. 7. Thedetailed description is, therefore, omitted.

As above-described, according to the present invention, (1) a metalfield plate is formed during formation of metal lead wire (eg.source/drain electrodes), and proper voltage is applied to it, so thatit is not required to take any separate additional manufacturing processfor forming a field plate, and to adjust it so as to hold low thresholdvoltage; (2) it is possible to obtain low conduction resistance and highbreakdown voltage by excluding a current distortion path; (3) it ispossible to reduce sharply the area occupied by elements in the systemdue to reduction of the drift area and the gate length; and (4) it ispossible to embody the most reliable high voltage MOSFET capable ofreducing the calorific power in chips compared with existing elements.

What is claimed is:
 1. A method for manufacturing the high voltageMOSFET, comprising a process forming the first and second conductivewells in a semiconductor substrate; process forming drift areas in thesaid first and second conductive wells; process growing an isolationmembrane on the substrate surface between the said first and secondconductive wells; process forming a gate insulation film; processforming a gate on the gate insulation film above the said first andsecond conductive wells; process forming low concentration n-and p-typedopant areas in the drift areas of the part adjacent to the said gate;process forming a buried diffusion area in the said first and secondconductive wells; process forming source/drain having a body contact ona side on the buried diffusion area in the said first and secondconductive wells; process forming the insulation film having a contactformed in such way that the source/drain surface is exposed, on theentire surface of the substrate including the said gate and isolationmembrane; process forming a metal film on the said insulation film; andprocess forming source/drain electrodes and metal field plates byetching the said metal film using a mask.
 2. The method formanufacturing the high voltage MOSFET according to claim 1, wherein thefirst conductive well is formed through a process diffusing p-typedopants after ion-injection.
 3. The method for manufacturing the highvoltage MOSFET according to claim 1, wherein said second conductive wellis formed through a process diffusing n-type dopants afterion-injection.
 4. The method for manufacturing the high voltage MOSFETaccording to claim 1, wherein the process forming low concentrationn-and p-type dopant areas in the drift area of a part adjacent to thesaid gate, further comprising a process forming the first mask layer onthe entire surface of the insulation film including the said gate;process etching the said first mask layer in such way that is exposed aspecified part of the insulation film on a side adjacent to the gate onthe drift area formed in the said second conductive well; procession-injecting low concentration n-type dopants in such exposed party bya self-align with the said gate, and then removing the said first masklayer; process forming the second mask layer on the entire surface ofthe gate insulation film including the said gate; process etching thesaid second mask layer in such way that is exposed a specified part ofthe insulation film on a side adjacent to the gate on the drift areaformed in the said first conductive well; process ion-injecting lowconcentration p-type dopants in such exposed part by a self-align withthe said gate, and then removing the said second mask layer; and processimplementing the diffusion.
 5. The method for manufacturing the highvoltage MOSFET according to claim 1, wherein the process forming asource/drain having a body contact formed on a side on the burieddiffusion area in the said first and second conductive wells, furthercomprises a process forming the first mask layer on the entire surfaceof the insulation film including the said gate; process etching the saidfirst mask layer in such way that are exposed specified parts of thesurface of insulation film on the buried diffusion area formed in thesaid first conductive well and the surface of insulation film on theburied diffusion area formed in the second conductive well; procession-injecting high concentration n-type dopants in such exposed parts,and then removing the said first mask layer; process forming the secondmask layer on the entire surface of the insulation film including thesaid gate; process etching the said second mask layer in such way thatare exposed specified parts of the surface of insulation film on theburied diffusion area formed in the said first conductive well, and thesurface of insulation film on the buried diffusion area formed in thesecond conductive well; process ion-injecting high concentration p-typedopants in such exposed parts, and then removing the said second masklayer; and process activating the said ion-injection area.
 6. The methodfor manufacturing the high voltage MOSFET according to claim 1, whereinthe gate is formed in the length of 1.2 μm to 1.5 μm.
 7. The method formanufacturing the high voltage MOSFET according to claim 1, wherein tothe said metal field plate is applied the gate voltage, or a constantvoltage separate from the gate voltage.
 8. The method for manufacturingthe high voltage MOSFET according to claim 1, wherein said high voltageMOSFET is formed so as to hold the value of breakdown voltage rangingfrom 20V to 600V.
 9. A method for manufacturing the high voltage MOSFET,comprising a process forming the first and second conductive wells in asemiconductor substrate; process forming the first and second p-typedrift areas in the said first conductive well; process forming the firstand second n-type drift areas in the said second conductive well;process growing an isolation membrane on the substrate surface betweenthe said first and second conductive wells; process growing field oxidesat both edges of the said first and second p-type drift areas and thesaid first and second n-type drift areas; process forming gateinsulation films; process forming gates on the gate insulation filmbetween the said first and second n-type drift areas, and on the gateinsulation film between the said first and second p-type drift areas;process forming low concentration n-and p-type dopants in the first andsecond p- and n-type drift areas of parts adjacent to the said gate;process forming the source/drain areas in the said first and second p-and n-type drift areas, and forming simultaneously body contacts on bothsides of the field oxide; process forming an insulation film having acontact formed in such way that the source/drain areas and body contactsurfaces are exposed, over the entire surface of the substrate includingthe said gate and isolation membrane; process forming a metal film onthe said insulation film; and process forming the source/drainelectrodes and the first and second metal field plates, by selecting andetching the said metal film.
 10. The method for manufacturing the highvoltage MOSFET according to claim 9, wherein said first conductive wellis formed through a process diffusing p-type dopants afterion-injection.
 11. The method for manufacturing the high voltage MOSFETaccording to claim 9, wherein said second conductive well if formedthrough a process diffusing n-type dopants after ion-injection.
 12. Themethod for manufacturing the high voltage MOSFET according to claim 9,wherein the process forming low concentration n- and p-type dopants inthe first and second p- and n-type drift areas in parts adjacent to thesaid gate, further comprises a process forming the first mask layer overthe entire surface of the insulation film including the said gate;process etching the said first mask layer in such way that are exposedspecified parts of the insulation films in the first and second n-typedrift areas on the left and right of lower part of the said gate;process ion-injecting low concentration n-type dopants in such exposedparts, and then removing the said first mask layer; process forming thesecond mask layer over the entire surface of the insulation filmincluding the said gate; process etching the said second mask layer insuch way that are exposed specified parts of the insulation film on thefirst and second p-type drift areas on the left and right of lower partof the said gate; process ion-injection low concentration p-type dopantsin such exposed parts, and then removing the said second mask layer; andprocess implementing the diffusion.
 13. The method for manufacturing thehigh voltage MOSFET according to claim 9, wherein the process formingthe source/drain areas in the said first and second p- and n-type driftareas, and forming simultaneously the body contacts on both sides of thefield oxide, further comprises a process forming the first mask layerover the entire surface of the insulation film including the said gate;process etching the said first mask layer in such way that are exposedspecified parts of the surfaces of the said first and second p-typedrift areas, and on the substrate surface on the left and right of thefield oxides formed at both edges of the first and second n-type driftareas; process ion-injecting high concentration p-type dopants in suchexposed parts, and then removing the said first mask layer; processforming the second mask layer over the entire surface of the insulationfilm including the said gate; process etching the said second mask layerin such way that are exposed specified parts of the surfaces of the saidfirst and second n-type drift areas, and on the substrate surface on theleft and right of the field oxides formed at both edges of the first andsecond p-type drift areas; process ion-injecting high concentrationn-type dopants in such exposed parts, and then removing the said secondmask layer; and process activating the said ion-injected areas.
 14. Themethod for manufacturing the high voltages MOSFET according to claim 9,wherein said gate is formed in length of 1.2 μm to 1.5 μm.
 15. Themethod for manufacturing the high voltage MOSFET according to claim 9,wherein to the said metal field plate is applied the gate voltage or aconstant voltage separate from the gate voltage.
 16. The method formanufacturing the high voltage MOSFET according to claim 9,characterized in that the said high voltage MOSFET is formed so as tohold the value of breakdown voltage ranging from 20V to 600V.
 17. Amethod for manufacturing the high voltage MOSFET, comprising a processforming the first conductive well in a semiconductor substrate; processforming a drift area in the said first conductive well; process forminga gate insulation film on the substrate in which the said drift area isformed, and then forming a gate on it; process forming low concentrationn-or p-type dopant area in the drift area of the part adjacent to thesaid gate; process forming a buried diffusion area in the said firstconductive well; process forming a source/drain having body contactformed on a side above the buried diffusion are in the said firstconductive well; process forming an insulation film having a contactformed in such way that the surface of the source/drain is exposed onthe substrate including the said gate; process forming a metal film onthe said insulation film; and process forming the source/drainelectrodes and metal field plates by etching the said metal film using amask.
 18. The method for manufacturing the high voltage MOSFET accordingto claim 17, wherein said first conductive well is formed through aprocess diffusing p- or n-type dopants after ion-injection.
 19. Themethod for manufacturing the high voltage MOSFET according to claim 17,wherein the process forming low concentration n- or p-type dopant areain the drift area of the part adjacent to the said gate, furthercomprises a process forming a mask layer over the entire surface of theinsulation film including the said gate; process etching the said masklayer in such way that is exposed a specified part of the insulationfilm on the side adjacent to the gate on the said drift area; procession-injecting low concentration n- or p-type dopants in such exposedpart by a self-align with the said gate; and then removing the said masklayer; and process implementing the diffusion.
 20. The method formanufacturing the high voltage MOSFET according to claim 17, wherein theprocess forming the source/drain having a body contact formed on a sideon the buried diffusion area in the said first conductive well, furthercomprises a process forming a mask layer over the entire surface of theinsulation film including the said gate; process etching the said masklayer in such way that the surface of the insulation film on the saidburied diffusion area is exposed; process ion-injection highconcentration n- or p-type dopants in such exposed part, and thenremoving the said mask layer; and process activating the saidion-injected area.
 21. The method for manufacturing the high voltageMOSFET according to claim 17, wherein said gate is formed in length of1.2 μm to 1.5 μm.
 22. The method for manufacturing the high voltageMOSFET according to claim 17, wherein to the said metal field plate isapplied the gate voltage or a constant voltage separate from the gatevoltage.
 23. The method for manufacturing the high voltage MOSFETaccording to claim 17, wherein said high voltage MOSFET is formed so asto hold the value of breakdown voltage ranging from 20V to 600V.
 24. Amethod for manufacturing the high voltage MOSFET, comprising a processforming the first conductive well in a semiconductor substrate; processforming the first and second drift areas in the said first conductivewell; process growing field oxides at both edges of the said first andsecond drift areas; process forming a gate insulation film; processforming a gate on the gate insulation film between the said first andsecond drift areas; process forming low concentration n- or p-typedopants in the first and second drift areas of parts adjacent to thesaid gate; process forming source/drain areas in the said first andsecond drift areas, and forming simultaneously body contacts on bothsides of the field oxide; process forming an insulation film having acontact formed in such way that are exposed the source/drain areas andbody contacts, over the entire surface of the substrate including thesaid gate; process forming a metal film on the said insulation film; andprocess forming source/drain electrodes and the first and second metalfield plates, by etching the said metal film using a mask.
 25. Themethod for manufacturing the high voltage MOSFET according to claim 24,wherein said first conductive well is formed through a process diffusingn- or p-type dopants after ion-injection.
 26. The method formanufacturing the high voltage MOSFET according to claim 24, wherein theprocess forming low concentration n- or p-type dopant areas in the firstand second drift areas of parts adjacent to the said gate, furthercomprises a process forming a mask layer over the entire surface of theinsulation film including the said gate; process etching the said masklayer in such way that are exposed specified parts of the insulationfilm on the first and second drift areas on the left and right of lowerparts of the said gate; process ion-injecting low concentration n- orp-type dopants in such exposed parts, and then removing the said masklayer; and process implementing the diffusion.
 27. The method formanufacturing the high voltage MOSFET according to claim 24, wherein theprocess forming source/drain areas in the said first and second driftareas, and forming simultaneously body contacts on both sides of thefield oxide, further comprises a process forming the first mask layerover the entire surface of the insulation film including the said gate;process etching the said first mask layer in such way that are exposedspecified parts of the substrate surface on the left/right of the fieldoxides formed at both edges of the said first and second drift areas;process ion-injecting high concentration n- or p-type dopants in suchexposed parts, and then removing the said first mask layer; processforming the second mask layer on the entire surface of the insulationincluding the said gate; process etching the said second mask layer insuch way that are exposed specified parts of the surfaces of the saidfirst and second drift areas; process ion-injecting high concentrationn- or p-type dopants in such exposed parts, and then removing the saidsecond mask layer; and process activating the said ion-injected areas.28. The method for manufacturing the high voltage MOSFET according toclaim 24, wherein said gate is formed in length of 1.2 μm to 1.5 μm. 29.The method for manufacturing the high voltage MOSFET according to claim24, wherein to the said metal field plate is applied the gate voltage ora constant voltage separate from the gate voltage.
 30. The method formanufacturing the high voltage MOSFET according to claim 24, whereinsaid high voltage MOSFET is formed so as to hold the value of breakdownvoltage ranging from 20V to 600V.